Flexible printed wiring board

ABSTRACT

A flexible printed wiring board includes a first conductor layer in the element mounting part adjacent to the top surface of the wiring board; a second conductor layer in the element mounting part adjacent to the bottom surface of the wiring board; and a third conductor layer between the first conductor layer and the second conductor layer, wherein the first and third conductor layers extend through and beyond the bending part, and the second conductor layer is absent in the bending part.

This application claims the benefit of Japanese Application No.2004-273669, filed on Sep. 21, 2004 in Japan, which is herebyincorporated by reference in its entirety.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to a flexible printed wiring board, and moreparticularly, to a flexible printed wiring board having an elementmounting part where circuit elements are mounted and a bending part tobe bent around a bending axis.

2. Discussion of the Related Art

Recent progress in information society invites rapid increase ininformation quantity, and rapid exchange and transmission of a largecapacity of information data are required. Consequently, integration inelectronic circuit elements has been improved, and enhancement inperformance, enhancement in function and enhancement in integration arein progress for electronic information apparatus. In such a trend,printed wiring boards used in electronic information apparatus are alsoundergoing enhancement in thinning, miniaturization, and intensifiedfunction, and various proposals have been made for flexible printedwiring boards as well. See Patent Document Nos. 1-5, listed below, forexample.

Patent Document No.1: Japanese Patent Laid-Open No. 1993-243741

Patent Document No.2: Japanese Patent Laid-Open No. 1994-216537

Patent Document No.3: Japanese Patent Laid-Open No. 1996-130351

Patent Document No.4: Japanese Patent Laid-Open No. 1996-125342

Patent Document No.5: Japanese Patent Laid-Open No. 1995-202358

Among such flexible printed wiring boards, with respect to those withmemory elements mounted thereon, a new structure with an increasedmemory capacity and speed has been proposed and put into practical use.For example, flexible printed wiring boards in which a chip size package(CSP) is mountable to both surfaces have been developed.

Such a flexible printed wiring board with CSPs mounted on both surfacesis arranged to have a layered structure throughout the substratesurface, as shown in FIGS. 15A and 15B, typically consisting of threeconductor layers (PT1 to PT3), two insulating layers (IN1 and IN2)isolating these conductor layers, and two coverlay layers (CL1 and CL2)(hereafter referred to as “related art example”). In addition, in aflexible printed wiring board of the related art example, memoryelements, for example, are mounted on both surfaces of an elementmounting part 60′ as shown in FIG. 15A. A bending part 70′ is bentaround the bending axis AX′, so that a motherboard connecting part 80′,which is formed at the top surface on the other end, is made in electriccontact with the motherboard MB (FIG. 15B). Consequently, the efficiencyin implementation of memory elements on a motherboard can be improved.

When the above described related art example of a flexible printedwiring board is bent along the bending axis, tensile stress is appliedto the outer side, while compressive stress is applied to the innerside. In addition, depending on the curvature at the time of bending,cracks may occur in the insulating layers. Therefore, it is preferableto decrease the curvature of bending; but the decrease in the curvatureof bending would impede high-density mounting of memory elements.

Therefore, currently, a flexible printed wiring board with improvedbending-withstanding properties is desired.

SUMMARY OF THE INVENTION

Accordingly, the present invention is directed to a flexible printedwiring board and a method of manufacturing the same that substantiallyobviate one or more of the problems due to limitations and disadvantagesof the related art.

An object of the present invention is to provide a flexible printedwiring board that has improved crack resistance.

Another object of the present invention is to provide a flexible printedwiring board that has an improved electronic continuity property.

Another object of the present invention is to provide a corelessthin-type flexible wiring board that has superior crack resistance andelectronic continuity properties.

Another object of the present invention is to provide a flexible printedwiring board that has an improved characteristic impedance.

Another object of the present invention is to provide an efficient andhigh-yield manufacturing method for the flexible printed wiring board ofthe present invention.

Another object of the present invention is to provide a both-surfacemountable flexible wiring board that enables high-density low-profilemounting of circuit elements on a host board.

Additional features and advantages of the invention will be set forth inthe description which follows and in part will be apparent from thedescription, or may be learned by practice of the invention. Theobjectives and other advantages of the invention will be realized andattained by the structure particularly pointed out in the writtendescription and claims hereof as well as the appended drawings.

To achieve these and other advantages and in accordance with the purposeof the present invention, as embodied and broadly described, accordingto one aspect of the invention, there is provided a flexible printedwiring board having an element mounting part configured to mount circuitelements on both top and bottom surfaces thereof and a bending part thatis to be bent around a bending axis that extends substantially inparallel with the wiring board, the top surface of the wiring boardbeing defined as a surface that comes to the outermost side when thebending part is bent around the bending axis, the bottom surface of thewiring board being defined as a surface that comes to the innermost sidewhen the bending part is bent around the bending axis, the flexibleprinted wiring board including a first conductor layer in the elementmounting part adjacent to the top surface of the wiring board; a secondconductor layer in the element mounting part adjacent to the bottomsurface of the wiring board; and a third conductor layer between thefirst conductor layer and the second conductor layer, wherein the firstand third conductor layers extend through and beyond the bending part,and the second conductor layer is absent in the bending part.

In another aspect, the present invention provides a flexible printedwiring board having three parts of an element mounting part, a foldingpart, and a board mounting part, which are laterally disposed in thatorder, the element mounting part being configured to mount circuitelements on both top and bottom surfaces of the wiring board, the boardmounting part being configured such that the top surface of the boardmounting part can be mounted on an external circuit board, the flexibleprinted wiring board being configured to be folded over at the foldingpart around a virtual folding line that extends substantially inparallel with the wiring board such that the top surface of the wiringboard faces an exterior when the flexible printed wiring board is bent,the flexible printed wiring board including: a bottom conductor patternin the element mounting part, the bottom conductor pattern beingconfigured to mount a first circuit element on the bottom surface of thewiring board through a first plurality of pads that are directly incontact with the bottom conductor pattern, the bottom conductor patternbeing absent in the folding part; a lower insulating layer on the bottomconductor pattern in all of said three parts, the lower insulating layerhaving a first plurality of via holes in the element mounting part; amiddle conductor pattern on the lower insulating layer in all of saidthree parts, portions of the middle conductor pattern being electricallyin contact with portions of the bottom conductor pattern via said firstplurality of via holes; an upper insulating layer on the middleconductor pattern in all of said three parts, the upper insulating layerhaving a second plurality of via holes in the element mounting part; anda top conductor pattern on the upper insulating layer in all of saidthree parts, the top conductor pattern in the element mounting partbeing configured to mount a second circuit element on the top surface ofthe wiring board through a second plurality of pads that are directly incontact with the top conductor pattern, the top conductor pattern in theboard mounting part being configured to mount the flexible printedwiring board to an external circuit board through a plurality of padsthat are directly in contact with the top conductor pattern, portions ofthe top conductor pattern being electrically in contact with portions ofthe middle conductor pattern via said second plurality of via holes.

In another aspect, the present invention provides a flexible wiringboard for mounting circuit elements on both surfaces on one end, theother end of the flexible wiring board being configured to be foldedover against said one end and configured to be mounted on an externalhost board to enable high-density low profile mounting of the circuitelements on the external host board, the flexible wiring board having athree-layered conductor structure on said one end so that the circuitelements can be mounted on both surfaces and having a two-layeredconductor structure in a fold-over portion at which the flexible wiringboard is to be folded over, the two layered conductor structure beingconfigured to provide for improved crack resistance at the fold-overportion.

In another aspect, the present invention provides a method formanufacturing a flexible printed wiring board having an element mountingpart configured to mount circuit elements on both top and bottomsurfaces thereof and a bending part that is to be bent around a bendingaxis that extends substantially in parallel with the wiring board, thetop surface of the wiring board being defined as a surface that comes tothe outermost side when the bending part is bent around the bendingaxis, the bottom surface of the wiring board being defined as a surfacethat comes to the innermost side when the bending part is bent aroundthe bending axis, the method including the steps of: forming a firstconductor layer in the element mounting part adjacent to the top surfaceof the wiring board; forming a second conductor layer in the elementmounting part adjacent to the bottom surface of the wiring board; andforming a third conductor layer between the first conductor layer andthe second conductor layer, wherein the first and third conductor layersextend through and beyond the bending part, and the second conductorlayer is absent in the bending part.

It is to be understood that both the foregoing general description andthe following detailed description are exemplary and explanatory, andare intended to provide further explanation of the invention as claimed.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings, which are included to provide a furtherunderstanding of the invention and are incorporated in and constitute apart of this application, illustrate embodiments of the invention andtogether with the description serve to explain the principle of theinvention. In the drawings:

FIGS. 1A and 1B illustrate a construction of a flexible printed wiringboard according to one embodiment of the present invention. FIG. 1A is aschematic perspective view. FIG. 1B is a side view.

FIG. 2 is a cross-sectional view of a flexible printed wiring boardaccording to an embodiment of present invention.

FIG. 3 is a schematic view illustrating the wiring direction of thesignal conductor pattern of a flexible printed wiring board according toan embodiment of the present invention.

FIGS. 4A and 4B are cross-sectional views illustrating manufacturingsteps of a flexible printed wiring board according to an embodiment ofthe present invention.

FIG. 5 is a schematic view for showing the directions of the warp andthe weft of the fiber-reinforced fabric in an insulating layer of aflexible printed wiring board according to an embodiment of the presentinvention.

FIGS. 6A and 6B are cross-sectional views illustrating manufacturingsteps of a flexible printed wiring board according to an embodiment ofthe present invention.

FIGS. 7A and 7B are cross-sectional views illustrating manufacturingsteps of a flexible printed wiring board according to an embodiment ofthe present invention.

FIGS. 8A and 8B are cross-sectional views illustrating manufacturingsteps of a flexible printed wiring board according to an embodiment ofthe present invention.

FIG. 9 is a cross-sectional view illustrating the manufacture of aflexible printed wiring board according to an embodiment of the presentinvention.

FIG. 10 is a cross-sectional view illustrating the manufacture of aflexible printed wiring board according to an embodiment of the presentinvention.

FIGS. 11A and 11B are cross-sectional views illustrating manufacturingsteps of a flexible printed wiring board according to an embodiment ofthe present invention.

FIG. 12 is a cross-sectional view illustrating a manufacturing step of aflexible printed wiring board according to an embodiment of the presentinvention.

FIGS. 13A and 13B are cross-sectional views illustrating manufacturingsteps of a flexible printed wiring board according to an embodiment ofthe present invention.

FIG. 14 is a sectional view showing a construction of a flexible printedwiring board according to an embodiment of the present invention.

FIGS. 15A and 15B illustrate a construction of a flexible printed wiringboard according to the related art. FIG. 15A is a schematic perspectiveview. FIG. 1B is a side view.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

A flexible printed wiring board according to a preferred embodiment ofthe present invention includes an element mounting part where circuitelements are mounted and a bending part to be bent around the bendingaxis. An inner conductor layer is formed on the inside of the board. Inthe element mounting part, element mounting part conductor layers areformed on both surfaces at outer sides of the wiring board,respectively, and in the bending part, a bending part conductor layer isformed only on the surface on the outer side that faces the exteriorwhen the wiring board is bent.

Here, in the above described element mounting part, the element mountingpart conductor layers are formed on both surfaces at the outer sides ofthe wiring board, respectively. In contrast, in the bending part, thebending part conductor layer is formed only on the side that faces theexterior when it is bent. Therefore, as compared with the related artexample described above, this configuration allows a reduction of thenumber of conductor layers in the bending part by one and the number ofcoverlay layer by one as well. Thus, by reducing the number of layers inthe bending part, the total thickness can be made thin as compared withthe related art flexible printed wiring boards. Consequently, when theflexible wiring board is bent at the same curvature, the stress isreduced, and therefore crack resistance is improved.

In addition, in the above described inner layer conductor layer, aconductor pattern that imparts a ground potential in terms ofalternating current may be formed. Thus, by forming a conductor patternto impart a ground potential in terms of alternating current as aninterior layer conductor layer, the characteristic impedance of a signalcircuit pattern (also referred to as “outer layer conductor pattern”)formed at the side that faces the exterior can be stabilized even whenthe flexible printed wiring board is bent along the bending axis.

In particular, in the inner conductor layer in the bending part, aconductor pattern to impart the ground potential in terms of alternatingcurrent may be formed in a plane pattern. In this case, signal lines ofthe outer layer conductor pattern in the bending part can be formed toconstitute a microstrip configuration, thereby further stabilizing thecharacteristic impedance.

The insulating layer formed between the above described conductorpatterns is preferably made of fiber-reinforced plastic. Suchfiber-reinforced plastic can include, for example, fiber-glassreinforced plastic (GFRP), carbon fiber reinforced plastic (CFRP) andthe like. More specifically, glass-fiber-reinforced epoxy,glass-fiber-reinforced polyester resin and the like may be used.

The thickness of the above described insulating layer is preferablyabout 25 μm to about 65 μm. Here, when the thickness of the abovedescribed insulating layer is less than about 25 μm, it is difficult toform an insulating layer with a uniform thickness, while when thethickness exceeds about 65 μm, preferable crack resistance may not beobtained.

In addition, the warp and the weft contained in the above describedfiber-reinforced plastic preferably extend in the directionsintersecting the direction of the above described bending axis by anangle of about 30° or more and about 60° or less. With thisconfiguration, when the wiring board is bent, the warp and the weftmechanically cooperate to improve crack resistance.

The signal conductor pattern formed in the above described bending partconductor layer preferably extends in the direction that is obliquelydisposed relative to the direction of the above described bending axis.This way, when the bending part is bent around the bending axis, thecrack-prevention performance of the signal conductor pattern is furtherimproved and the occurrence of cracks can be further reduced.

Incidentally, the above described circuit element can be a memoryelement. In this case, high density implementation of memory elements,which has been desired more and more in recent years, becomes possible.

One advantage of the present invention is that a flexible printed wiringboard with improved crack resistance can be provided.

FIGS. 1A and 1B shows a configuration of a flexible printed wiring board10 according to an embodiment of the present invention. FIG. 1A is aperspective view thereof FIG. 1B is an XZ side view of the flexibleprinted wiring board 10 which is bent at a bending part along thebending axis AX and is attached to a motherboard MB.

In this embodiment, on the upper surface (i.e., the surface facing inthe +Z direction) of the flexible printed wiring board 10 shown in FIG.1A, pads 44U₁ to 44U_(N) for mounting a circuit element 100A and pads47L₁ to 47L_(N) are provided, and pads 44U₁ to 44U_(N) and pads 47L₁ to47L_(N) are electrically connected, respectively, for example.

In addition, although not shown in FIG. 1A, on the lower surface (i.e.,the surface facing in the −Z direction) of the flexible printed wiringboard 10, pads 45U₁ to 45U_(N) for mounting a circuit element 100B areformed at locations that correspond to the pads 44U₁ to 44U_(N). Forexample, these pads 45U₁ to 45U_(N) and pads 47L₁ to 47L_(N) are alsoelectrically connected, respectively.

Here, in the present embodiment, both circuit element 100A and circuitelement 100B can be memory elements of the same type.

The flexible printed wiring board 10 of the present embodiment isemployed as follows. As shown in FIG. 1, the element 100A is mounted onthe upper surface and the element 100B is mounted on the lower surfaceof the flexible printed wiring board 10. In mounting the flexibleprinted wiring board 10 on a host board, such as a mother board, theflexible printed wiring board 10 is bent around the bending axis AX, andis mounted on the motherboard MB, as shown in FIG. 1B. In this case, thecircuit element 100B of the flexible printed wiring board 10 is bondedvia a bonding layer 90 to a surface of the folded-over portion of theflexible printed wiring board, which is opposite to the surface havingthe pads 47L₁ to 47L_(N).

FIG. 2 shows an XZ cross-sectional view of the flexible printed wiringboard 10 of the present embodiment. As shown in FIG. 2, the flexibleprinted wiring board 10 of the present embodiment includes an elementmounting parts 60A and 60B (also referred to as “element mounting part60” collectively) where the circuit elements 100A and 100B as describedabove are to be mounted, a bending part 70 where this flexible printedwiring board is bent around the bending axis AX, and a motherboardconnecting part 80, which is to be connected to a motherboard.

In addition, the flexible printed wiring board 10 includes, in theelement mounting part 60, (a) an insulating layer 13, (b) an insulatinglayer 17U formed on a surface in the +Z direction side of the insulatinglayer 13, (c) an insulating layer 20U, which is the outmost layer formedon a surface in the +Z direction side of the insulating layer 17U and(d) an insulating layer 22 formed on a surface in the −Z direction sideof the insulating layer 13. Here, the insulating layers 20U and 22respectively function as coverlay layers. Since the insulating layer 22is not formed in the bending part 70, the bending part 70 includes thelayers (a) to (c), but does not include the insulating layer 22 (d).

In addition, the flexible printed wiring board 10 includes (e) aconductor pattern 34U′ formed on the surface in the +Z direction of theinsulating layer 13, (f) a first conductor pattern 36U′, which is asignal line pattern, formed on the surface in the +Z direction of theinsulating layer 17U and (g) a second conductor pattern 33L, which is asignal line pattern, formed on the surface in the −Z direction of theinsulating layer 13 in the element mounting part 60.

Here, this conductor pattern 34U′ includes a conductor pattern to imparta ground potential in terms of alternating current (hereinafter alsoreferred to as “ground pattern” or “GNP”), and additionally includescircuit patterns formed at via holes. The GNP may be formed in a solidpattern to cover substantially the entire area at the bending part 70.

The first conductor pattern 36U′ includes a power supply pattern aswell. Here, in the bending part 70, the first conductor pattern 36U′ isarranged to extend in the direction that intersects the bending axis AX(the Y direction) at an angle θ. In this embodiment, as shown in FIG. 3,conductor patterns P₁ to P_(N) (which electrically connect the pads44U₁-44U_(N) to 47L₁-47L_(N), respectively, for example) are notdisposed parallel to the X direction, but intersect a virtual lineextending in the Y direction at angle θ.

With the above-described configuration, because the ground potentialpattern of the conductor pattern 34U′ in the bending part 70 is formedto cover the substantially entire area, signal lines of the signal linepattern 36U′ in the bending part 70 can be made very thin, therebyforming a microstrip configuration. In this case, the characteristicimpedance can be stabilized.

Moreover, the flexible printed wiring board 10 includes (h) via holes inthe insulating layers 13 and 17U, respectively, for providinginterconnections among conductor pattern 36U′ (including GNP), the firstconductor pattern 36U′, and the second conductor pattern 33L.

Here, although not depicted in FIG. 2, via holes are also provided inthe motherboard connecting part 80 to provide interconnections betweenthe GNP (conductor pattern 34U′) and the first conductor pattern 36U′.

Pads 44U₁ to 44U_(N) for mounting circuit element 100A are formed to bein contact with the first conductor pattern 36U′. In addition, pads 45U₁ to 45 U_(N) for mounting another circuit element 100B are formed tobe in contact with the second conductive pattern 33L.

Here, the length of wiring pattern connecting the pads 44U_(j) (j=1 toN) with the pads 47L_(j) and the length of wiring pattern connecting thepads 45U_(j) with the pads 47L_(j) can be made respectively the same toimplement equal length wiring.

As the material of the insulating layers 13 and 17U, epoxy resin,glass-fiber-reinforced epoxy resin (hereinafter also referred to as“glass epoxy” or “prepreg”) obtained by impregnating epoxy resin intoglass fiber, glass-fiber-reinforced polyimide resin obtained byimpregnating polyimide resin into glass fiber, and the like can be used.In manufacturing the flexible printed wiring board of the presentembodiment, glass epoxy is preferably used in terms of dimensionalstability, mass productivity and thermal stability. Here, the insulatinglayers 13 and 17U may be formed of the same material selected from theabove described materials, or may be formed with mutually differentmaterials.

In addition, as for the insulating layers 20U and 22 forming coverlaylayers, polyimide resin coated with epoxy-based adhesive and the likecan be used. In consideration of flexibility, heat resistance,insulating properties, and corrosion resistance, polyimide resin ispreferable.

As the material for the conductor patterns 33L, 34U′, and 36U′,conductive metal such as copper, aluminum, stainless steel and the likecan be used. In particular, in consideration of workability, copper ispreferably used.

Next, manufacturing steps of the flexible printed wiring board 10 willbe described. At first, a supporting member (hereinafter also referredto as “reinforcing layer”) 11 shown in FIG. 4A is prepared. Here, as thesupporting member 11, from the viewpoint of ease in handling duringmanufacturing steps, prepreg is preferably used. Specifically, GHPL830(manufactured by Mitsubishi Gas Chemical Company, Inc.), E679(manufactured by Hitachi Chemical Co., Ltd.), R1661 (manufactured byMatsushita Electric Works, Ltd.) and the like can be used. In terms ofcosts as well as dimensional stability, R1661 is preferable.

Next, a conductor film with a carrier (31L, 32U), an insulating layer12, and a conductor foil 32L are prepared. The conductor film with acarrier (31L, 32U) is to be laminated on the surface in the −Z directionof the supporting member 11. The insulating layer 12 is to be laminatedon the surface in the −Z direction of the conductor film with a carrier(31L, 32U). The conductor foil 32L is to be laminated on the surface inthe −Z direction of the insulating layer 12. In addition, a conductorfilm with a carrier (31U, 33L), an insulating layer 13, and a conductorfoil 33U are prepared. The conductor film with a carrier (31U, 33L) isto be laminated on the surface in the +Z direction of the supportingmember 11. The insulating layer 13 is to be laminated on the surface inthe +Z direction of the conductor film with a carrier (31U, 33L). Theconductor foil 33U is to be laminated on the surface in the +Z directionof the insulating layer 13.

The above-mentioned conductor film with a carrier can be manufactured bypressing a conductor film (32U, 33L) to adhere onto the surface of acarrier member (31L, 31U). The conductor film (32U, 33L) is attached tothe carrier member by an adhesive, such as an adhesive that containsbenzotriazole or benzotriazole derivative. For example, VERZONE (SF-310,manufactured by DAIWA KASEI K. K.) and the like can be used so that theresulting film can be delaminated at a later time. In addition,commercially available products may be appropriately selected and used.

Such commercially available products allow subsequent delamination of acarrier member from the conductor film. The examples include Micro-thin(manufactured by Mitsui Mining and Smelting Co., Ltd.), XTR(manufactured by Olin Brass), and UTC-Foil (manufactured by METFOILSAB).

Prepreg is preferably used as the insulating layers 12 and 13. Ascommercially available products, prepreg with a thickness of about 25 μmto about 100 μm, such as GHPL830 (manufactured by Mitsubishi GasChemical Company, Inc.), E679 (manufactured by Hitachi Chemical Co.,Ltd.), and R1661 (manufactured by Matsushita Electric Works, Ltd.) andthe like can preferably be used in terms of the required thickness ofthe final product. In light of the thinning trend and improvement incrack resistance of flexible printed wiring boards, those with athickness of about 25 μm to about 65 μm are more preferable.

Here, as shown in FIG. 5, in the prepreg used as the insulating layers12 and 13, the direction of the warp WA (and therefore the weft WE) ofthe prepreg is preferably arranged to obliquely intersects the directionof the bending axis AX (i.e., the Y direction).

The intersection angle φ is not particularly limited. However, from theviewpoint of improvement in crack resistance at the time of bending. Theangle φ is preferably about 30° to about 60°. When the angle φ is about45°, it provides the greatest prevention effect on crack occurrence ininsulating layers.

Referring to FIG. 4A, the conductor film with a carrier (31L, 32U) islaminated on the reinforcing layer 11 so that the surface in the −Zdirection of the reinforcing layer 11 and the surface in the +Zdirection of the conductor film with a carrier (31L, 32U) are broughtinto contact. The insulating layer 12 is formed on the conductor filmwith a carrier (31L, 32U) so that the surface in the −Z direction of theconductor film with a carrier (31L, 32U) and the surface in the +Zdirection of the insulating layer 12 are brought into contact.

The conductor film with a carrier (31U, 33L) is laminated on thereinforcing layer 11 so that the surface in the +Z direction of thereinforcing layer 11 and the surface in the −Z direction of theconductor film with a carrier (31U, 33L) are brought into contact. Theinsulating layer 13 is formed on the conductor film with a carrier (31U,33L) so that the surface in the +Z direction of the conductor film witha carrier (31U, 33L) and the surface in the −Z direction of theinsulating layer 13 are brought into contact.

The reinforcing layer 11 and the two insulating layers laminated asshown in FIG. 4A are pressed under predetermined conditions, forexample, at about 185° C. under a pressure of about 40 kg/m² for aboutan hour, to produce a laminated body (FIG. 4A).

Subsequently, a CO₂ laser process is performed to form a via hole 41U.The opening 41U is formed so as to reach the surface in the +Z directionof the conductor layer 33L from the surface in the +Z direction of theinsulating layer 13 (see FIG. 4B).

To form the opening 41U, first, a conductor layer 33U is formed on theinsulating layer 13, and a region of the conductor layer 33U at whichthe via hole 41U will be formed on the surface in the +Z direction ofthe conductor layer 33U undergoes blackening. Subsequently, this regionhaving undergone blackening is irradiated with a laser beam having apredetermined energy from the above to form the opening 41U.

In forming a via hole 41L on the surface in the −Z direction of theinsulating layer 12, the similar process is implemented (see FIG. 4B).

The conductor layers 33U and 32L are formed by pressing the conductorfilm 33U and the conductor film 32L to adhere onto the surface in the +Zdirection of the insulating layer 13 and the surface in the −Z directionof the insulating layer 12, respectively.

Copper foil and the like may be used as the conductor films 33U and 32L.A conductor film with a carrier can be used to form a very thin layer ofthe conductor films 33U and 32L. In such a case, the conductor film witha carrier is first laminated on the corresponding insulating layer, andthereafter the carrier member is pealed off to leave the thin conductorfilm on the insulating layer.

Here, it is preferable to use a conductor film with a carrier having aconductor film thickness of about 3 μm to about 9 μm, such as Micro-Thin(manufactured by Mitsui Mining and Smelting Co., Ltd.), XTR(manufactured by Olin Brass), UTC-Foil (manufactured by METFOILS AB) orthe like.

Referring to FIG. 6A, the remaining upper surface in the +Z direction ofthe conductor pattern 33U, the side surface of the opening 41U and thebottom surface of the opening 41U (that is, the exposed surface in the+Z direction of the conductor pattern 33L inside the opening 41U)undergo metal plating so that a plated opening is formed and a conductorfilm 34U is formed. Similarly, the remaining conductor pattern 32L, theside surface of the opening 41L, and the bottom surface of the opening41L undergo metal plating so that a plated opening is formed and aconductor film 34L is formed.

The plating can be performed with a copper plating bath with acomposition shown in Table 1 below. TABLE 1 Copper sulfate plating bathcomposition Plating bath Name of compound Quantity (g/L) Copper sulfate125 to 250 Sulfuric acid  30 to 100

Subsequently, referring to FIG. 6B, a resist layer is formed on theentire upper surface of the laminated body and is patterned by a knownlithography process to form a resist pattern 16U, which covers theplated via hole 41U′. Similarly, a resist pattern 16L is formed on thelower surface in the −Z direction of the laminated body to cover theplated via hole 41L′.

As the resist layer, an acrylic dry film resist, such as HW440(manufactured by Hitachi Chemical Co., Ltd.), for example, can be used.Moreover, NIT1025 (manufactured by Nippon Synthetic Chemical IndustryCo., Ltd.), SA-50 (manufactured by DuPont) and the like can also beused.

Subsequently, by employing a tenting process using an etching solutionincluding copper (II) chloride or the like, the solder delaminationprocess using a metal resist, or the micro-etching process suitable forfine pattern forming or the like, etching is performed until the surfacein the +Z direction of the insulating layer 13 and the surface in the −Zdirection of the insulating layer 12 are exposed (see FIG. 7A).

As a result, a conductor pattern 34U′ is formed on the surface in the +Zdirection of the insulating layer 13. Also, a plated non-through viahole 41U′ for electrically connecting the conductor pattern 34U′ to theconductor layer 33L is formed. Likewise, on the surface in the −Zdirection of the insulating layer 12, a conductor pattern 34L′ isformed, and a plated non-through via hole 41L′ for electricallyconnecting the conductor pattern 34L′ to the conductor layer 32U isformed.

Next, an insulating layer 17U is formed on the surface in the +Zdirection of the insulating layer 13, and an insulating layer 17L isformed on the surface in the −Z direction of the insulating layer 12.Here, the insulating layers 17U and 17L may be formed by laminationpressing through pin lamination. For these insulating layers 17U and17L, a material similar to that used for the insulating layers 12 and 13can be used. Subsequently, conductor layers 35U and 35L are formed onthe surface in the +Z direction of the insulating layer 17U and on thesurface in the −Z direction of the insulating layer 17L, respectively(see FIG. 7B).

The conductor layers 35U and 35L are formed by pressing the conductorfilm 35U and the conductor film 35L to adhere onto the surface in the +Zdirection of the insulating layer 17U and the surface in the −Zdirection of the insulating layer 17L, respectively.

Copper foil and the like can be used as the conductor films 35U and 35L.A conductor film with a carrier may be used to form a very thin layer ofthe conductor films 35U and 35L. In such a case, the conductor film witha carrier is laminated on the corresponding insulating layer, andthereafter the carrier member is pealed off to leave the thin conductorfilm on the insulating layer.

Here, it is preferable to use a conductor film with a carrier having aconductor film thickness of about 3 μm to about 9 μm, such as Micro-Thin(manufactured by Mitsui Mining and Smelting Co., Ltd.), XTR(manufactured by Olin Brass), UTC-Foil (manufactured by METFOILS AB) orthe like.

Subsequently, using a process similar to the process for forming theabove-described openings 41U and 41L, an opening 42U is formed on theinsulating layer 17U and an opening 42L is formed on the insulatinglayer 17L (see FIG. 8A). Subsequently, using a plating process similarto the plating process described above, conductor films 36U and 36L areformed (see FIG. 8B). Thereafter in a manner similar to the mannerdescribed above, formation of a resist layer, and etching and removal ofthe resist layer are performed to form conductor pattern 36U′ and 36L′(see FIG. 9).

Next, referring to FIG. 10, an ink is printed and hardened to form acoverlay layer 20U having openings 43U in a manner similar to thephotolithography method. Likewise, the cover layer 20L having openings43L is formed. Here, polyimide resin such as CKSE (manufactured byNIKKAN INDUSTRIES Co., Ltd.), for example, can be used to form thecoverlay layers 20U and 20L. In the alternative, instead of an ink, aresist film may be laminated to form the coverlay layers.

Consequently, laminated bodies 10U and 10L are formed on the respectivesurfaces of the reinforcing layer 11 (see FIG. 10). Here, as shown inFIG. 10, the laminated body 10U includes the conductor layer 33L, theinsulating layer 13, the insulating layer 17U and the coverlay layer20U. The insulating layers 13 and 17U are respectively provided with viaholes for inter-layer connection.

As shown in FIG. 10, the laminated body 10L includes the conductor layer32U, the insulating layer 12, the insulating layer 17L and the coverlaylayer 20L. The insulating layers 12 and 17L are respectively providedwith via holes for inter-layer connection.

The following steps will be described with reference to the laminatedbody 10U. The laminated body 10L will be processed in the same orsimilar manner. The laminated body 10U formed on the surface in the +Zdirection of the reinforcing layer 11 is separated from the reinforcinglayer 11 at the interface between the carrier member 31U and theconductor layer 33L (see FIG. 11A).

Subsequently, using the conductor layer 33L, which is formed on thesurface in the −Z direction of the insulating layer 13, as a platinglead, nickel plating is carried out on portions of the upper surface ofthe laminated body 10U that are not covered by the coverlay layer 20U(FIG. 11B). Here, the nickel plating can be conducted with a platingbath shown in Table 2 under the following conditions: pH 4 to 5, liquidtemperature of 40 to 60° C. and current density of approximately 2 to 6A/dm². TABLE 2 Nickel electrolytic plating bath composition Plating bathName of compound Quantity (g/L) Nickel sulfate Approximately 300 Nickelchloride Approximately 50 Boric acid Approximately 40

Subsequently, gold plating can be performed on the portion that hasundergone nickel plating using a plating bath with a composition shownin Table 3 under the following conditions: liquid temperature of 20 to25° C. and current density of 0.2 to 1.0 A/dm². Here, in FIG. 11B, thetwo plated layers are illustrated as one layer. TABLE 3 Au electrolyticplating bath composition Plating bath Name of compound Quantity (g/L)Gold 10 Sodium cyanide 30 to 35 Ammonia 50 to 60

After completion of the above-described plating process, an ink isprinted and hardened on the conductor layer 33L provided on the surfacein the −Z direction of the laminated body 10U to form a resist layer 21Lin a matter similar to the photolithography method (FIG. 12). Here, AUSseries (manufactured by TAIYO INK MFG CO., LTD.) and DSR series(manufactured by TAMURA Corporation), for example, can be used to formthe resist layer.

Here, the resist layer 21L can be formed only in the element mountingpart where circuit elements will be mounted on the surface in the −Zdirection of the conductor layer 33L. Alternatively, it may be formed onthe entire surface except the bending part.

Subsequently, by disposing soldering paste onto the openings 43U byscreen printing and subsequently performing a solder reflow process, orby using the solder ball direct mounting method or the like, pads 44U₁to 44U_(N) are formed (FIG. 13A).

Next, as shown in FIG. 13A, the uncovered portion of conductor layer 33Lis etched to expose the surface in the −Z direction of the insulatinglayer 13. Then, the resist layer 21L is removed by making it come upusing NaOH, thereby exposing the resulting conductor pattern 33L.

Subsequently, as shown in FIG. 13B, a coverlay layer 22 is formed so asto cover the surface in the −Z direction of the exposed insulating layer13 and the conductor layer 33L, and openings 44L are formed in a mannersimilar to that used for forming the openings 43U.

Subsequently, by a process similar to the process described above, pads45U₁ and 45U_(N) are formed at the openings 44L, thereby completing acoreless thin type flexible printed wiring board 10.

The manufacturing process of the flexible printed wiring board 10described above provides an excellent yield.

Moreover, in the above-described manufacturing method, the conductorlayer 33L is used as the plating lead for plating, and this conductorlayer 33L is processed after plating to form a conductor pattern.Therefore, the step of providing a plating lead and the step of pealingit off, which are required in the conventional art, is no longerrequired. This expedites the production of flexible printed wiringboards.

The laminated body 10L formed on the surface in the −Z direction of thereinforcing layer 11 undergoes the same process as the above-describedprocess for the laminated body 10U so that a flexible printed wiringboard having the same structure as the laminated body 10U ismanufactured.

In the above described embodiment, the ground pattern included in theconductor pattern 34U′ in the bending part 70 is formed as a solidpattern that substantially covers the entire area over which signallines are formed. Alternatively, a power source pattern that imparts aground potential in terms of alternating current may be formed in asimilar solid pattern. In this case, signal lines by the outer layerconductor pattern 36U′ can be formed to constitute a microstripconfiguration in the bending part 70. Therefore, the characteristicimpedance can be further stabilized.

Moreover, as for the metal plating used for the above-describedmanufacture of the flexible printed wiring board, nickel plating andsubsequent gold plating were employed. However, a different combinationof the same or different metal materials may be used in the plating.

Here, in the above described embodiment, the equal length wiring wasrealized by providing element mounting parts 60A and 60B on theleft-hand side. Alternatively or in addition, as shown in FIG. 14, theother side (the right side) of the flexible printed wiring board 10 maybe provided with the circuit pattern 33L and the coverlay layer 22 toimplement equal length wiring.

The flexible printed wiring board of the present embodiment is useful asa thin type flexible printed wiring board. In particular, the flexibleprinted wiring board of the present embodiment has a stable in-lineimpedance when high-speed multi-pin logic LSIs and the like are mountedthereon and has excellent crack resistance.

Moreover, the method of manufacturing a flexible printed wiring board ofthe present embodiment is suitable for manufacturing a thin typeflexible printed wiring board with an excellent yield.

The flexible printed wiring board 10 manufactured as described above isbent (folded over) along the bending axis AX after electronic circuitchips, such as the memory elements 100A and 100B, are mounted onto theelement mounting parts 60A and 60B, respectively. Then, the surface inthe −X direction of the memory element 100B is affixed to the surface inthe −Z direction of the flexible printed wiring board 10 with anadhesive. Subsequently, as shown in FIG. 1B, the motherboard MB and themotherboard connecting part 80 are electrically connected so that theflexible printed wiring board with the circuit chips is mounted onelectronic information apparatus.

Working examples of the present invention will now be described indetail. However, the present invention will not be limited by theseexamples in any ways.

Manufacture of Flexible Printed Circuit Boards in Working Examples 1 to10

As the reinforcing layer 11, R1661 (manufactured by Matsushita ElectricWorks, Ltd.) was used. In forming the conductor film with a carrier(31L, 32U) to be laminated on the lower surface of the reinforcing layer11, the conductor foils 32L and 33U to be laminated on the insulatinglayers 12 and 13, respectively, and the conductor film with a carrier(31U, 33L) to be laminated on the upper surface of the reinforcing layer11, Micro-thin (manufactured by Mitsui Mining and Smelting Co., Ltd.)was used. For these conductor films with a carrier, XTR (manufactured byOlin Brass) or UTC-Foil (manufactured by METFOILS AB) may also be usedin place of Micro-thin.

As the insulating layers 12 and 13, GHPL830 (manufactured by MitsubishiGas Chemical Company, Inc.) was used. Alternatively, E679 (manufacturedby Hitachi Chemical Co., Ltd.) or R1661 (manufactured by MatsushitaElectric Works, Ltd.) may also be used. The thickness of the prepregranged from about 25 μm to about 65 μM (as shown in Table 7 below).

The prepregs used as the insulating layers 12 and 13 were arranged sothat the direction of the fabric of the warp WA (and therefore the weftWE) of the prepreg obliquely intersects a line extending in the Ydirection to form an angle ranging from 30° to 60°, depending on WorkingExamples (see Table 7 below).

The reinforcing layer 11 and the insulating layers 12 and 13 werelaminated as shown in FIG. 4A and were pressed under a pressure of about40 kg/m² at about 185° C. for about one hour to form a laminated body.

Then the conductor layers 33U and 32L were formed. Subsequently,portions on the conductor layers 33U and 32L over the insulating layers12 and 13 at which the non-through via holes 41L and 41U should beformed were blackened and irradiated with a CO₂ laser beam to form theopenings 41U and 41L, respectively (FIG. 4B).

Next, the remaining surface of the conductor pattern 33U and theinterior of the opening 41U as well as the remaining surface of theconductor pattern 32L and the interior of the opening 41L underwentmetal plating with a plating bath using the composition shown in Table 4below to form the conductor films 34U and 34L (FIG. 6A). TABLE 4 CopperSulfate Plating Bath Composition Plating bath Name of compound Quantity(g/L) Copper sulfate 125 to 250 Sulfuric acid  30 to 100

Subsequently, an acrylic dry film resist HW440 (manufactured by HitachiChemical Co., Ltd.) was laminated on the whole surface of conductor film34U, and the resist is patterned by a known lithography method to form aresist pattern 16U defining regions where a conductor pattern should beformed (FIG. 6B).

In a similar fashion, a resist pattern 16L was formed on the surface ofconductor film 34L to define regions where a conductor pattern should beformed (FIG. 6B).

Subsequently, by employing a tenting process using copper (II) chloride,the solder delamination process using a metal resist, or themicro-etching process suitable for fine pattern forming, etching wasperformed until the surface in the +Z direction of the insulating layer13 and the surface in the −Z direction of the insulating layer 12 wereexposed (FIG. 7A).

Accordingly, a conductor pattern 34U′ was formed on the surface in the+Z direction of the insulating layer 13, and a plated non-through viahole 41U′ for electrically connecting the conductor pattern 34U′ to theconductor layer 33L was formed. Likewise, on the surface in the −Zdirection of the insulating layer 12, a conductor pattern 34L′ wasformed, and a plated non-through via hole 41L′ for electricallyconnecting the conductor pattern 34L′ to the conductor layer 32U wasformed (FIG. 7A).

Next, by lamination pressing through pin lamination, an insulating layer17U was formed on the surface in the +Z direction of the insulatinglayer 13, and an insulating layer 17L was formed on the surface in the−Z direction of the insulating layer 12 (FIG. 7B). Subsequently, bypressing Micro-Thin (with thickness of approximately 5 μm, manufacturedby Mitsui Mining and Smelting Co., Ltd.) to adhere onto the surface inthe +Z direction of the insulating layer 17U and onto the surface in the−Z direction of the insulating layer 17L, respectively, and by pealingoff the carrier member, conductor layers 35U and 35L were formed (FIG.7B).

Subsequently, using a process similar to the process for forming theopenings 41U and 41L, an opening 42U was formed in the insulating layer17U and an opening 42L was formed in the insulating layer 17L (FIG. 8A).Subsequently, using a plating process that is the same as or similar tothe above-described process for forming the conductor layers 34U and34L, conductor films 36U and 36L were formed (FIG. 8B). Then, using aprocess that is the same as or similar to the above-described processfor forming the conductor pattern 34U′ and 34L′, conductor patterns 36U′and 36L′ were formed (FIG. 9).

Subsequently, an ink is printed and hardened to form a coverlay layer20U having openings 43U in a manner similar to the photolithographymethod. Likewise, the cover layer 20L having openings 43L is formed onthe opposite side. As a result, laminated bodies 10U and 10L were formedon the respective surfaces of the reinforcing layer 11 (FIG. 10).

As described above, the laminated body 10U formed on the surface in the+Z direction of the reinforcing layer 11 was separated from thereinforcing layer 11 at the interface between the carrier member 31U andthe conductor layer 33L (FIG. 11A).

Subsequently, using the conductor layer 33L, which has been formed onthe surface in the −Z direction of the insulating layer 13, as theplating lead, nickel plating was carried out on the whole surface of theportions that were not covered by the coverlay layer 20U using a platingbath with the composition shown in Table 5 under the followingconditions: pH 4 to 5, liquid temperature of 40 to 60° C. and currentdensity of approximately 2 to 6 A/dm². TABLE 5 Nickel ElectrolyticPlating Bath Composition Plating bath Name of compound Quantity (g/L)Nickel sulfate Approximately 300 Nickel chloride Approximately 50 Boricacid Approximately 40

Subsequently, gold plating was performed on the portions that haveundergone nickel plating using a plating bath with the composition shownin Table 6 under the following conditions: liquid temperature of 20 to25° C. and current density of 0.2 to 1.0 A/dm² (FIG. 11B). TABLE 6 AuElectrolytic Plating Bath Composition Plating bath Name of compoundQuantity (g/L) Gold 10 Sodium cyanide 30 to 35 Ammonia 50 to 60

After the completion of the above-described plating process, a resistlayer 21L was formed on the conductor layer 33L provided on the surfacein the −Z direction of the laminated body 10U using AUS series(manufactured by TAIYO INK MFG. CO., LTD.). In stead of AUS series, DSRseries (manufactured by TAMURA Corporation) (FIG. 12) may be used.

Subsequently, by disposing a soldering paste onto the openings 43U byscreen printing and by solder reflowing, pads 44U₁ to 44U_(N) wereformed. Instead of using screen printing, the solder ball directformation method may be used to form the pads.

Next, etching was performed to expose the surface in the −Z direction ofthe insulating layer 13 and the resist layer 21L was removed by makingit come up using NaOH of 20 to 40 g/L, thereby forming conductor pattern33L (FIG. 13A).

Subsequently, a coverlay layer 22 was formed so as to cover the surfacein the −Z direction of the exposed insulating layer 13 and the surfacein the −Z direction of the conductor layer 33L, and openings 44L wereformed in a manner similar to that used for forming the openings 43U(FIG. 13B).

Subsequently, in a manner similar to that used for forming pads 44U₁ to44U_(N), pads 45U₁ and 45U_(N) were formed inside the openings 44L,thereby completing working examples of a coreless thin type flexibleprinted wiring board 10 according to the present invention. WorkingExamples 1-10 differ among themselves in terms of the following variousmanufacturing and dimensional parameters: the thickness of insulatinglayers 13 and 17L, the width of the conductor pattern 36U′, the angle φof the prepreg fiber of the insulating layers 13 and 17L (FIG. 5), andthe angle φ of the conductor pattern 36U′ in the bending part 70 (FIG.3). These parameters are listed in Table 7.

Here, in Working Examples 1 to 10 and Reference Examples 1 to 4 (whichwill be described below), the coverlay layer 22 was formed only in theportion 60B where electronic circuit chips are mounted on the surface inthe −Z direction of the conductor layer 33L (see FIG. 2).

Bending Test and Continuity Test

Bending tests and continuity tests were carried out for flexible printedwiring boards of Working Examples 1 to 10 with impedance of 50Ω (designvalue), which were manufactured as described above. The MIT (flexuralfatigue resistance) test of JIS5016 was adopted as the bending test, andoccurrence of cracks in insulating layers and conductor layers wereexamined. In addition, the continuity test was conducted with a TCT(thermo cycle test) tester, and the continuity was examined afterpredetermined numbers of repetition of the thermal cycle consisting ofraising the temperature from −55° C. to 125° C. in 30 minutes andlowering the temperature in the reverse manner. The continuity wasevaluated at 50 cycles and 100 cycles. The lost continuity results wereindicated as NG. The test results for Working Examples 1-10 are shown inTable 7.

Manufacture of Flexible Printed Circuit Boards of Comparative Examples 1to 5

Comparative Examples 1 to 5 were manufactured and tested. In ComparativeExamples 1 to 5, the conductor pattern 33L was formed in the bendingpart 70 as well. Thus, the number of conductor layers in the bendingpart 70 was three (3). Comparative Examples 1-4 differ among themselvesin terms of the fiber directional angle φ of the prepreg in theinsulating layer 17U and 13 and the bending angle φ of the conductorpattern 36U′. Otherwise, Comparative Examples were manufactured in thesame way as in the manufacture of the flexible printed wiring boards ofWorking Examples 1 to 10. The above described bending tests as well asthe continuity tests were conducted with respect to Comparable Examples1-4. The results are shown in Table 8.

Manufacture of Flexible Printed Circuit Boards of Reference Examples 1to 4

The flexible printed wiring boards of Reference Examples 1 and 2 weremanufactured in the same way as in the manufacture of the flexibleprinted wiring boards of Working Examples 1 to 10 except that the angleφ of the warp and the weft of the glass fiber in the insulating layers17L and 13 relative to the bending axis was set to 25° and 65°,respectively, and that the bending angle θ of the conductor pattern 36U′was set to 0°. These parameters and the results of the bending andcontinuity tests are listed in Table 8.

In addition, the flexible printed wiring boards of Reference Examples 3and 4 were manufactured in the same way as in the manufacture of theflexible printed wiring boards of Working Examples 1 to 10 except thatthe thickness of the insulating layers 17L and 13 was set to 100 μm, theangle φ of the warp and the weft of the glass fiber in the insulatinglayers 17L and 13 relative to the bending axis was set to 45°, and thatthe bending angle θ of the conductor pattern 16U′ was set to 0°. Theseparameters and the results of the bending and continuity tests arelisted in Table 8. TABLE 7 Status of Thickness Bending crack occurrenceContinuity test Conductor layer counts Conductor layer of insulatingAngle of angle of at the time of bending Number Implementation BendingWidth Thickness layer glass fiber pattern Impe- Insulating Conductor ofcycles Classification surface part (μm) (μm) (μm) (φ) (θ) dance* layerlayer 50 100 Example 1 3 2 50 20 40 35 90 50 None None OK OK Example 2 32 50 20 40 45 90 50 None None OK OK Example 3 3 2 50 20 40 55 90 50 NoneNone OK OK Example 4 3 2 50 20 40 60 45 50 None None OK OK Example 5 3 250 20 40 30 30 50 None None OK OK Example 6 3 2 50 20 40 45 45 50 NoneNone OK OK Example 7 3 2 30 20 25 45 45 50 None None OK OK Example 8 3 230 20 25 45 90 50 None None OK OK Example 9 3 2 80 20 65 45 45 50 NoneNone OK OK Example 10 3 2 80 20 65 45 90 50 None None OK OK*Design value (Ω)

TABLE 8 Status of Insulating Bending crack occurrence Continuity testConductor layer counts Conductor layer layer Angle of angle of at thetime of bending Number Implementation Bending Width Thickness Thicknessglass fiber pattern Impe- Insulating Conductor of cycles Classificationsurface part (μm) (μm) (μm) (φ) (θ) dance* layer layer 50 100 Comp. Ex.1 3 3 50 20 40  0 0 50 Occurred Occurred NG — Comp. Ex. 2 3 3 50 20 4015 0 50 Occurred Occurred NG — Comp. Ex. 3 3 3 50 20 40 45 0 50 NoneOccurred NG — Comp. Ex. 4 3 3 50 20 40 15 45  50 Occurred Occurred NG —Comp. Ex. 5 3 3 50 20 40 25 45  50 Occurred None NG — Ref. Ex. 1 3 2 5020 40 25 0 50 None None NG — Ref. Ex. 2 3 2 50 20 40 65 0 50 None NoneOK NG Ref. Ex. 3 3 2 120  20 100  45 0 50 None None OK NG Ref. Ex. 4 3 2120  20 100  45 0 50 None None OK NG*Design value (Ω)

As shown in Table 8, as for the flexible printed wiring boards ofComparative Examples 1 to 5, occurrence of crack was observed in bendingtests. Also in continuity tests, electrical continuity was already lostat 50 cycles.

In any of flexible printed wiring boards of Reference Examples 1 to 4,occurrence of crack was not observed at the time of bending. However, inthe continuity test, the flexible printed wiring board of ReferenceExample 1 already lost electrical continuity at 50 cycles.

As described above, it was found that a decrease in the number ofconductor layers in the bending part reduces occurrence of cracks at thetime of bending. In addition, it was found that by angularly offsettingthe direction of the glass fiber in insulating layers relative to thebending axis, it is possible to form a conductor pattern that canmaintain continuity after 50 cycles.

For each of the Working Examples 1 to 10, cracks did not occur in theinsulating layers in bending tests, and electrical continuity wasmaintained after 100 cycles in the continuity test. Thus, it was foundthat a decrease in the number of conductor layers in the bending partcoupled with an angular configuration of either or both of the glassfiber in the insulating layers and the signal line conductor patternfurther improves crack resistance.

As described above, thin-type flexible printed wiring boards of WorkingExamples 1 to 10 excelled in crack resistance.

The flexible printed wiring board of the present invention is useful asa thin-type flexible printed wiring board and is particularly suitablefor miniaturizing high-speed and large-capacity memories and the like.

Moreover, the method of manufacturing the flexible printed wiring boardof the present invention is suitable for manufacturing thin-typeflexible printed wiring boards that have superior crack resistance andhas an excellent yield.

It will be apparent to those skilled in the art that variousmodifications and variations can be made in the present inventionwithout departing from the spirit or scope of the invention. Thus, it isintended that the present invention cover the modifications andvariations of this invention provided they come within the scope of theappended claims and their equivalents.

1. A flexible printed wiring board having an element mounting partconfigured to mount circuit elements on both top and bottom surfacesthereof and a bending part that is to be bent around a bending axis thatextends substantially in parallel with the wiring board, the top surfaceof the wiring board being defined as a surface that comes to theoutermost side when the bending part is bent around the bending axis,the bottom surface of the wiring board being defined as a surface thatcomes to the innermost side when the bending part is bent around thebending axis, the flexible printed wiring board comprising: a firstconductor layer in the element mounting part adjacent to the top surfaceof the wiring board; a second conductor layer in the element mountingpart adjacent to the bottom surface of the wiring board; and a thirdconductor layer between the first conductor layer and the secondconductor layer, wherein the first and third conductor layers extendthrough and beyond the bending part, and the second conductor layer isabsent in the bending part.
 2. The flexible printed wiring boardaccording to claim 1, wherein the third conductor layer includes aconductor pattern configured to impart a ground potential.
 3. Theflexible printed wiring board according to claim 1, further comprising:a first insulating layer between the first conductive layer and thethird conductive layer; and a second insulating layer between the secondconductive layer and the third conductive layer, wherein the first andsecond insulating layer are made of fiber-reinforced plastic.
 4. Theflexible printed wiring board according to claim 3, wherein thethickness of each of the first and second insulating layers is about 25μm to about 65 μm.
 5. The flexible printed wiring board according toclaim 3, wherein said fiber-reinforced plastic includes warps and wefts,and the warps or the wefts of said fiber-reinforced plastic extend in adirection that forms an angle of about 30° to about 60° with respect tothe direction of the bending axis.
 6. The flexible printed wiring boardaccording to claim 1, wherein the second conductor layer includes aplurality of signal lines that extends in a direction obliquely disposedrelative to the direction of the bending axis.
 7. The flexible printedwiring board according to claim 1, wherein the element mounting part isconfigured to mount one or more memory elements.
 8. A flexible printedwiring board having three parts of an element mounting part, a foldingpart, and a board mounting part, which are laterally disposed in thatorder, the element mounting part being configured to mount circuitelements on both top and bottom surfaces of the wiring board, the boardmounting part being configured such that the top surface of the boardmounting part can be mounted on an external circuit board, the flexibleprinted wiring board being configured to be folded over at the foldingpart around a virtual folding line that extends substantially inparallel with the wiring board such that the top surface of the wiringboard faces an exterior when the flexible printed wiring board is bent,the flexible printed wiring board comprising: a bottom conductor patternin the element mounting part, the bottom conductor pattern beingconfigured to mount a first circuit element on the bottom surface of thewiring board through a first plurality of pads that are directly incontact with the bottom conductor pattern, the bottom conductor patternbeing absent in the folding part; a lower insulating layer on the bottomconductor pattern in all of said three parts, the lower insulating layerhaving a first plurality of via holes in the element mounting part; amiddle conductor pattern on the lower insulating layer in all of saidthree parts, portions of the middle conductor pattern being electricallyin contact with portions of the bottom conductor pattern via said firstplurality of via holes; an upper insulating layer on the middleconductor pattern in all of said three parts, the upper insulating layerhaving a second plurality of via holes in the element mounting part; anda top conductor pattern on the upper insulating layer in all of saidthree parts, the top conductor pattern in the element mounting partbeing configured to mount a second circuit element on the top surface ofthe wiring board through a second plurality of pads that are directly incontact with the top conductor pattern, the top conductor pattern in theboard mounting part being configured to mount the flexible printedwiring board to an external circuit board through a plurality of padsthat are directly in contact with the top conductor pattern, portions ofthe top conductor pattern being electrically in contact with portions ofthe middle conductor pattern via said second plurality of via holes. 9.The flexible printed wiring board according to claim 8, wherein the topconductor pattern including a plurality of signal lines in the foldingpart, and wherein the middle conductor pattern includes a ground patternconfigured to impart a ground potential, and the ground pattern has asolid pattern that covers substantially an entire area over which saidplurality of signal lines are disposed in the folding part.
 10. Theflexible printed wiring board according to claim 9, wherein saidplurality of signal lines of the top conductor pattern extends inparallel with each other in a direction that forms an angle of about 30°to about 90° with respect to the direction of the virtual folding line.11. The flexible printed wiring board according to claim 8, wherein theupper and lower insulating layers are made of a fiber-reinforced resin.12. The flexible printed wiring board according to claim 11, whereinsaid fiber-reinforced resin includes warps and wefts of fiber, and thewarps or the wefts of said fiber-reinforced resin extend in a directionthat forms an angle of about 0° to about 60° with respect to thedirection of the virtual folding line.
 13. The flexible printed wiringboard according to claim 8, wherein the thickness of each of the upperand lower insulating layers is about 25 μm to about 65 μm.
 14. Theflexible printed wiring board according to claim 8, wherein the topconductor pattern includes a plurality of signal lines in the foldingpart that extends in parallel with each other in a direction that formsan angle of about 30° to about 90° with respect to the direction of thevirtual folding line, wherein the middle conductor pattern includes aground pattern configured to impart a ground potential, and the groundpattern has a solid pattern that covers substantially an entire areaover which said plurality of signal lines are disposed in the foldingpart, wherein the upper and lower insulating layers are made of afiber-reinforced resin having warps and wefts, and each have a thicknessof about 25 μm to about 65 μm; and the warps or the wefts of saidfiber-reinforced plastic extend in a direction that forms an angle ofabout 0° to about 60° with respect to the direction of the virtualfolding line.
 15. The flexible printed wiring board according to claim8, further comprising a first coverlay layer covering the bottomconductor pattern in the element mounting part, wherein the coverlaylayer is absent in the folding part.
 16. The flexible printed wiringboard according to claim 15, further comprising a second coverlay layercovering the top conductor pattern in all of said three parts.
 17. Theflexible printed wiring board according to claim 8, wherein the bottomconductor pattern is further disposed under the lower insulating layerin the board mounting part, wherein the lower insulating layer has athird plurality of via holes in the board mounting part, and portions ofthe middle conductor pattern are electrically in contact with portionsof the bottom conductor pattern in the board mounting part via saidthird plurality of via holes, wherein the upper insulating layer has afourth plurality of via holes in the board mounting part, and portion ofthe top conductor pattern are electrically in contact with portions ofthe middle conductor pattern via said fourth plurality of via holes. 18.The flexible printed wiring board according to claim 8, wherein each ofthe first and second pluralities of pads has a multi-layered structure.19. A flexible wiring board for mounting circuit elements on bothsurfaces on one end, the other end of the flexible wiring board beingconfigured to be folded over against said one end and configured to bemounted on an external host board to enable high-density low profilemounting of the circuit elements on the external host board, theflexible wiring board having a three-layered conductor structure on saidone end so that the circuit elements can be mounted on both surfaces andhaving a two-layered conductor structure in a fold-over portion at whichthe flexible wiring board is to be folded over, the two layeredconductor structure being configured to provide for improved crackresistance at the fold-over portion.
 20. A method for manufacturing aflexible printed wiring board having an element mounting part configuredto mount circuit elements on both top and bottom surfaces thereof and abending part that is to be bent around a bending axis that extendssubstantially in parallel with the wiring board, the top surface of thewiring board being defined as a surface that comes to the outermost sidewhen the bending part is bent around the bending axis, the bottomsurface of the wiring board being defined as a surface that comes to theinnermost side when the bending part is bent around the bending axis,the method comprising the steps of: forming a first conductor layer inthe element mounting part adjacent to the top surface of the wiringboard; forming a second conductor layer in the element mounting partadjacent to the bottom surface of the wiring board; and forming a thirdconductor layer between the first conductor layer and the secondconductor layer, wherein the first and third conductor layers extendthrough and beyond the bending part, and the second conductor layer isabsent in the bending part.